Dentistry Section of the Dept. S.Bi.BI.T of the University of Parma joined in cooperation. Compared to the above mentioned
techniques, photogrammetry offers three advantages: -
no further radiographic imaging are necessary, after those for planning the treatment
- optical images do not suffer from blooming effects
Koong, 2012 caused by the high reflectivity of the titanium of the implants in CT images, that make it
difficult to align accurately the pre- and post- intervention images;
- the CT machines currently available have a spatial
voxel resolution of about 25 micrometres.
1.2 Parameters used to compare master and copy
To evaluate metrically by photogrammetry the differences between the planning designed with the help of CT images and
implemented in the surgical mask and the actual placement realized on the patient, two moulds models each carrying the
implants are compared see Figures 3a and 3b. The former is obtained directly from the surgical mask master model,
inserting the implants in the guides and then by enclosing them in a gypsum mould; the latter is produced from the dental
impression of the patient and therefore shows the actual position of the implants after the surgery copy model.
Figure 3a. Master model
Figure 3b. Copy model To compare the two models and evaluate the discrepancies in
the implant position, different parameters can be taken into account. In the literature on this topic, most authors use the four
parameters shown in Figure 4. The implant’s head of the master
model is the reference point; the implant’s symmetry axis of the
master model is the reference axis. The discrepancies between the positions of the two heads and the two apices of the implant
are expressed in two components: along the reference axis and perpendicular to it; moreover, the angle between the reference
axis and the implant axis in the copy model should be determined.
Unlike TC imaging, a direct measurement of the discrepancy at the apex of the implant by photogrammetry is obviously
impossible. If the implant can be considered rigid enough, so that no shape deformation occurs, it is possible to compute the
discrepancy from the other parameters. 1: Dperp =
discrepancy at the implant’s head in the
plane perpendicular to the reference axis
2: discrepancy at the
implant’s apex in the plane perpendicular to
the reference axis 3: Dlong= axial
discrepancy: distance between the master
head and the projection of the copy
head on the reference axis
4: angle between the implant axes
Figure 4. Parameters used to describe the differences between the master and the copy model
The paper is organized as follows: Section 2 describes and discuss how the geometric parameters of Figure 4 can be
recovered from photogrammetric measurements, referring them to the same coordinate system; Section 3 illustrates the design
and implementation of a photogrammetric measurement protocol; finally, Section 4 describes the simulated and
experimental results to test the system and its capability to identify out-of-tolerance discrepancies between the planned and
actual implant positions.
2. DETERMINATION OF THE PARAMETERS FOR
